An optical inspection apparatus for a semiconductor wafer is an apparatus for detecting a foreign matter(s) on the wafer, thereby to manage a number of the foreign materials. Because of difference of sizes between the foreign matters to be managed, depending on a manufacturing process of the semiconductor, in the inspection apparatus is provided a function of changing an optical magnification or power. If enlarging the optical power, it is possible to detect the foreign matters, being small much more, on the other hand, a field of view, which can be detected by a sensor, also comes to small, and therefore, throughput goes down to small. Accordingly, on the semiconductor manufacturing line, in particular, limiting to a process or a step necessitating a fine or minute management, an inspection is conducted while enlarging the optical power. A detecting system of the inspection apparatus is built up with an objective lens and an imaging lens, and change of the optical power is carried out, in general, by changing the focus distance of the imaging lens.
On a line for manufacturing a large amount of wafers is applied plural numbers of optical inspection apparatuses. A permissible value of pieces of the foreign matters is already determined, in advance, on each of the manufacturing processes or steps, and when the number of pieces exceeds that permissible number, then a countermeasure for is taken, for example, cleaning of the apparatus, etc. Herein, if there is difference in sensitivities (i.e., sizes of detectable foreign matters) between an optical inspection apparatuses “A” and “B”, it is necessary to determine or set up the permissible for each optical inspection apparatus; i.e., bringing about a large barrier on the operation thereof. The sensitivity of the optical inspection apparatus depends, largely, upon an imaging capacity of lenses of the detecting system. Accordingly, for the purpose of reducing the sensitivity of the optical inspection apparatus, it is necessary to execute the management of an imaging lens, as a unit, in particular, the management of the wavefront aberration thereof.
As a method for measuring the wavefront aberration of a lens is already known a method of applying Shack-Hartmann waverfront sensor (hereinafter, “Shack-Hartmann sensor”) therein, as is described in the Patent Document 1 (Japanese Patent Laying-Open No. 2004-14764). The Shack-Hartmann sensor is a sensor for photographing the wavefront (i.e., phase distribution) of lights entering upon the sensor, dividing and condensing that by means of an array lens, in the form of an image of alignment of plural numbers of spots, on a 2-dimensional sensor, and it calculates a wavefront aberration coefficient from the position shift of the spot alignment.
A method for calculating the wavefront aberration or the wavefront aberration coefficient is already disclosed in the Patent Document 1 (Japanese Patent Laying-Open No. 2004-14764) mentioned above or the Patent Document 2 (Japanese Patent Laying-Open No. 2006-30016). Measurement of the wavefront aberration by means of Shack-Hartmann sensor is advantageous in the following aspects; (1) it is hardly influenced by change of the environment, such as, temperature change of an air within an optical path, or vibrations, etc., (2) it is applicable also to a local change of the wavefront, being equal or larger than the measurement wavelength, which is generated on an spherical lens, etc., comparing to the method by an interferometer, being a main current conventionally. In this prior art, an object of measurement is a projection lens in a photolithography system. On the other hand, the lens of the detection system of the optical inspection apparatus is made up with the objective lens and the imaging lens.
As was mentioned above, for the purpose of changeability of the optical power, the focus distance of the imaging lens is changed. This may be achieved by exchanging the imaging lens, or applying a zoom lens to it. Although the wavefront aberration can be measured with using the objective lens and the imaging lens as one (1) set, like the projection lens, but there is no knowing that the aberration obtained is a result of cancelling a plus aberration generated on the objective lens by a minus aberration of the imaging lens. In this instance, if changing it to an imaging lens having the different focus distance, there is a possibility that the aberration changes largely. Accordingly, it is preferable to measure the aberration of the objective lens, as a unit. However, since the objective lens, as a unit, forms no image (i.e., an infinite system), it is necessary to adopt other measuring method, having the structure different from that shown in the prior arts mentioned above.
Also in a laser machining apparatus for use of the printed-circuit board, similar to the objective lens in the optical inspection apparatus for use of the semiconductor, an fθ lens of the indefinite system is applied. In this apparatus, parallel lights are deflected by a galvano-mirror, which is provided at a position of a pupil of the fθ lens, to enter on the fθ lens, and thereby scanning on the printed-circuit board by a condensed light beam. The fθ lens is a lens, being given with distortion thereon, intentionally, so that a beam position is determined on the printed-circuit board by a product fθ of the deflection angle of lights upon the galvano-mirror and the focus distance of the fθ lens. Since also the fθ lens is not the imaging lens, it is necessary to adopt other measuring method, differing from that of the conventional imaging type. In the laser machining apparatus, since the aberration of the fθ lens gives an influence upon a machining configuration at each scanning position of the condensed light beam, for the purpose of obtaining a uniform machining configuration within a region of the scanning, it is necessary to conduct the management upon the aberration (i.e., the wavefront aberration).
With the wavefront aberration measurement, applying the Shack-Hartmann sensor mentioned above therein, measurement is made on the aberrations, including the aberrations of the lens array of a measurement optical system and/or a relay lens, other than the aberration of the lens, i.e., a measuring object. Then, in the Patent Document 1 (Japanese Patent Laying-Open No. 2004-14764) is disclosed a method for calculating the aberration of the measurement optical system, i.e., by subtracting the data of the lens, i.e., the measuring object, as a unit, from data including the aberration of the measurement optical system, while having conducted the measurement of the lens, i.e., the measuring object, as a unit thereof, with using a separate means, such as, an interferometer, etc.
On the other hand, in the Patent Document 2 (Japanese Patent Laying-Open No. 2006-30016) is disclosed a method for cancelling the aberration of the measurement optical system therefrom, so as to calculate the aberration of only the lens, i.e., the measuring object; by measuring only the lens, i.e., the measuring object, two (2) times, while changing a posture thereof, such as, rotating, etc., and obtaining a difference for each measurement value.